The present invention relates generally to the treatment of a fluid. More particularly, it concerns an oriented structure for treating a fluid.
The use of biofilters or filters for water treatment and air treatment has been known for a long time. Already known in the art of water treatment, there is a reactor filled with porous material allowing water to flow either upwardly through the porous material in a submerging mode or downwardly in a percolating or submerging mode. In a submerging mode, the system consists of a solid-liquid bi-phase system while in a percolating mode, the system consists of a solid-liquid-gas tri-phase system. Depending on the goals of the treatment, the solid phase can be used for retaining particulate material, fixing micro-organisms and settling chemical reactions. The following review of the prior art is more specifically directed to filtering systems using percolating and tri-phase biofilters for treating waste water. However, some of the described phenomena can yet be applied to bi-phase reactors in a non-exclusive manner.
Equation 1 shows the relation existing between the three phases (solid, liquid, gas) in terms of hold-up corresponding to a fraction of the total volume of the reactor occupied by each of these phases.
1=xcex5s+xcex5L+xcex5gxe2x80x83xe2x80x83(1)
The solid hold-up, or xcex5s, can be subdivided in three components.
xcex5s=xcex5sm+xcex5sb+xcex5spxe2x80x83xe2x80x83(2)
where xcex5sm corresponds to the fraction of solid volume occupied by the filtering material, xcex5sb corresponds to the fraction of solid volume occupied by the biomass and xcex5sp corresponds to the fraction of volume occupied by the particulate materials retained in the trickling bed.
The liquid hold-up, or xcex5L, can be subdivided in two components:
xcex5L=xcex5Ls+xcex5Ldxe2x80x83xe2x80x83(3)
where xcex5Ld corresponds to the fraction of liquid volume occupied by the liquid in movement or flowing down and xcex5Ls corresponds to the fraction of liquid volume occupied by the static liquid held up in the trickling bed.
In the same way, gaseous hold-up, or xcex5g can be subdivided in two components, that is a static component (xcex5gs) and a dynamic component (xcex5gd):
xcex5g=xcex5gs+xcex5gdxe2x80x83xe2x80x83(4)
Taking as a reference the simple percolation sand filter, a very widespread technology (described for example in WO9700770 corresponding to CA 2,247,519, FR 2,745,195 in the name of Eparco), one can observe, with time, a clogging phenomena in the upper part of the trickling bed, caused by the accumulation of neo-formed biomass (xcex5sb) and of particulate materials (xcex5sp). This clogging phenomena in the upper part limits the flow of gaseous fluids and liquids (increase of xcex5s to the detriment of xcex5L and xcex5g). This decrease of gaseous fluids flowing towards the bottom can cause, in the gaseous hold-up (xcex5g) of the trickling bed located under the upper part, a limitation in oxygen. Such limitation can cause a slowing-down, even a complete stop of the oxidation reactions taking place in this part of the trickling bed. On its side, the limitation of the liquid flow on the surface of the bed causes a reduction of the hydraulic conductivity of the trickling bed which in turn can cause a decrease in the water volume than can be treated in a given time.
On the other hand, the operation of the reactor in a percolating mode can have the effect of increasing the liquid hold-up (xcex5L) in the lower part of the trickling bed, such increase resulting from a capillary phenomenon. The proportion of the static (xcex5Ls) and dynamic (xcex5Ld) fractions of the liquid hold-up (xcex5L) in this part of the bed can vary depending on whether the reactor is fed or not. This increase in the liquid hold-up (xcex5L) causes a reduction of the gaseous hold-up (xcex5g) in the lower part of the trickling bed, thereby limiting the gaseous exchange with the part located above the lower part. In turn, this can cause in the upper part, in the gaseous hold-up (xcex5g) of the trickling bed, a limitation in oxygen. This limitation can also lead to a slowing-down, even a complete stop, of the oxidation reactions taking place in this zone.
Finally, resulting from the water energy dissipated, many filtering materials operated in percolation mode undergo a compaction effect which leads to a decrease of the gaseous hold-up (xcex5g).
To increase the efficiency in terms of the oxidation capacity or the lifetime of trickling biofilters, different strategies, used alone or in combination (sometimes with contrary effects) have been adopted until today. Most of the times, these strategies involve the use of bulk and non-oriented materials.
Good gaseous and liquid flows towards the bottom (reduction of xcex5s) of the trickling bed can be maintained by vertically spreading the clogging zone appearing in the upper portion of the trickling filter. One solution to reach this goal is to recirculate a portion of the final effluent of the trickling bed into the trickling bed. Such recirculation leads to a dilution and an increase of the interstitial liquid velocity. This practice is however limited by the hydraulic loading capacity of the filtering material and by the fact that the increase of the superficial liquid velocity leads to an increase of the liquid hold-up (xcex5Ld) to the detriment of the gaseous hold-up (xcex5g). Such limitation has to be compensated in some cases by the use of a mechanically forced aeration provided by a fan or compressor. Moreover, such operation often implies using expensive pumps and valves.
The vertical Spreading of the clogging zone can also be increased by adjusting or changing different factors related to the porosity and porometry of the filter structure. To do so, one can mix more or less homogeneously components of different granulometry so as to obtain a structure with a larger and more extended porometry, as in Canadian patent no. 2,009,752. Others choose a structure comprising elements having an important percentage of empty voids interconnected inside and between each element. Polyurethane bodies are often used for this type of filter structure with two levels of porosity, as for example in Canadian patent application no. 2,139,554. These modifications are limited by the manufacturing or installation costs of the filtering medium and/or by the loss of efficiency caused by the reduction of the residence time of the liquid to be treated in the reactor (reduction of xcex5L).
Finally, it is possible to increase the spreading of the clogging zone while at the same time keeping a good purification capacity, and that, by changing the porometry of the filter structure and by introducing unit separators therein for providing a hydraulic discontinuity at different levels. The superimposition of distinct layers induces an upward capillary flow at those different levels, thereby allowing an increase in liquid hold-up (xcex5L) and, at the same time, an increase in the residence time of the liquid to be treated; and that, despite a more open porometry (Canadian patent no. 2,171,279). However, the spreading of the clogging zone which results from an increase of the liquid hold-up (xcex5L) by a hydraulic failure is obtained to the detriment of the gaseous hold-up (xcex5g). This situation could require the use of a mechanically forced aeration or the frequent replacement of the filter structure.
Other examples of prior art filtration systems are given in U.S. Pat. Nos. 4,101,423; 4,490,072; 4,745,716; 4,574,541; 4,639,165; 4,917,536; 4,815,892; 4,925,342; 4,983,068; 5,114,582; 5,232,429; 5,273,818; 5,5,624,204; 5,776,567; 5,827,430; 5,980,748; 6,048,131; 6,077,376; 4,880,333; 5,609,947; 5,096,591; 4,465,594; CA2,009752; CA2,139,554; and CA2,171,279.
An object of the present invention is to provide an improved structure for the treatment of fluids.
Another object is to provide a structure aiming to better control the different biological and physical phenomena related to the accumulation or exchange processes taking place in the structure.
More particularly, the present invention provides an oriented structure for treating a fluid, the structure comprising an inlet end for receiving a fluid to be treated and an outlet end opposite the inlet end and from which a treated fluid is discharged, the fluid being allowed to flow into the structure from the inlet end to the outlet end defining a flowing direction. The oriented structure also comprises a juxtaposition of layers of porous material having a different porosity, each layer spanning from the inlet end to the outlet end of the structure whereby in use the layers are oriented generally parallel to the flowing direction of the fluid. The structure is characterized in that the juxtaposition of layers comprises layers of a first material having a first porosity chosen so as to create a zone of dynamic hold-up of the fluid alternating with layers of a second material having a second porosity chosen so as to create a zone of static hold-up of the fluid, thereby promoting internal exchanges of fluid between the zone of dynamic hold-up and the zone of static hold-up.
One understands that the treatment of a fluid encompasses the filtration as well as the biofiltration of a fluid. It can also encompass the treatment by adsorption and/or absorption according to the type of material used.
The fluid that can be treated with a structure according to the present invention can be a liquid or a gas, more preferably it is a liquid and most preferably it is waste water
By juxtaposition of layers, one understands the alternation of layers as well as the inclusion of different layers of materials as it will be more fully described hereinafter.
The juxtaposition of the materials with different porosity can be done vertically, horizontally or obliquely.
According to another aspect, the present invention also provides an oriented structure for treating a liquid, the structure comprising:
a top side opposite a bottom side;
an inlet in the top side for receiving a liquid to be treated and an outlet in the bottom side from which a treated liquid is discharged, the liquid being allowed to flow into the structure from the inlet to the outlet end defining a flowing direction;
a vertical juxtaposition of layers of fiber textile alternating with layers of peat, each layer spanning from the top side to the bottom side of the structure whereby in use the layers are oriented generally parallel to the flowing direction of the liquid.
According to another aspect, the invention provides a reactor for treating a liquid, the reactor comprising:
a chamber with an upper portion and a lower portion;
a liquid inlet in the upper portion for introducing in the chamber a liquid to be treated and a liquid outlet in the lower portion for discharging from the chamber a treated liquid; and
at least one oriented structure as defined above mounted within the chamber, each layer of the at least one oriented structure spanning generally vertically within the chamber.
According to a still further aspect, the invention provides a method for manufacturing an oriented structure as defined above, comprising the steps of:
a) providing two mats of a first porous material having a porosity chosen so as to create a zone of dynamic hold-up of the fluid; and a second porous material made of a particulate material having a porosity chosen so as to create a static hold-up of the fluid;
b) forming a multilayer structure by covering one of said two mats with a layer of the second material and covering the layer of the second material with the other one of said two mats; and
c) rolling up the multilayer structure.